This invention relates to the electrodeposition of a metal, preferably copper, from aqueous acidic baths. More particularly this invention relates to an aqueous acidic bath for the electrodeposition of copper containing additives which provide leveled copper electrodeposits. Still more particularly this invention relates to the use of such baths to produce printed circuit boards.
A large number of agents have been described in the art for use in electroplating baths alone or in combination to improve the quality of the electrodeposit in terms of brightness, surface smoothness, hardening, leveling and to increase the lower limiting current density of deposition. The use of such agents in aqueous, acidic, copper plating baths for the preparation of printed circuits is described in Chapter 7 of the "Printed Circuits Handbook", Second Edition, 1979, McGraw-Hill Book Company, edited by Clyde F. Coombs, Jr., and in particular Sections 18 and 19. In Section 18, Coombs indicates that additives to acid copper sulfate plating baths can be effective in grain refinement, leveling, and hardening and as a brightener or a means of increasing the current density range. The term "leveled" denotes a copper deposit whose surface is smoother than its substrate. The term "bright" indicates that the formed electrodeposit is characterized by having a highly reflective surface gloss over most of its surface. Generally leveling and brightness vary with the current density at the cathode, all other factors such as copper salt concentration, pH, type of acid, temperature etc. being equal. As the current density decreases brightness of the electrodeposit tends to decrease often diminishing to a haze. The strength of leveling also varies with current density. Coombs indicates that such additives include glue, peptone, resorcinol, thiourea, molasses, gum Arabic as well as proprietary compositions.
A variety of brightening and leveling additives are disclosed in U.S. Pat. Nos. 3,502,551; 4,376,685 and 4,555,315 and the patents cited therein.
U.S. Pat. No. 3,502,551 discloses the use of polyvinyl amines in acid copper baths with oxygen-containing high-molecular compounds and organic thio compounds to obtain high-gloss copper precipitates with increased leveling effect.
U.S. Pat. No. 4,376,685 discloses acid copper electroplating baths containing as brightening and leveling additives an alkylated polyalkyleneimine obtained as a reaction product of a polyalkyleneimine with an epihalohydrin and an alkylating agent; an organic sulfosulfonate; a polyether and optionally a thioorganic compound.
U.S. Pat. No. 4,555,315 discloses copper plating baths for high speed electroplating wherein the bath contains as essential additives a polyether compound; an organic divalent sulfur compound, a reaction product of polyethyleneimine and an alkylating agent; and a partial adduct of a tertiary alkyl amine with polyepichlorohydrin to form a polyquaternary amine.
Although existing additives are useful in acid copper electroplating baths, leveler compositions are complex reaction products whose constitution and activity may vary from batch to batch. Consequently, there is a need in the printed circuit plating industry for leveler additives with consistent activity.